Cast and thermoplastic polyurethane compositions based on the reaction of polyisocyanates with polymeric diols are well known for use as elastomers, adhesives, sealants, elastomeric surface coatings, and coatings for metals and plastics. Cast elastomers are often used by molders to make high temperature resistant elastomers for specialty applications because the crosslinking produced by the amine or multifunctional polyol curing agents provides good resistance to creep and flow at high temperatures. However, cast elastomers based on traditional soft segments such as polyester or polyether diols typically have poor resistance to hydrolysis.
Polyurethane cast elastomers are typically made by one of two processes: one-shot, or prepolymer. The one-shot process is a single step process in which the isocyanate, polydiene diol, amine curing agent, and optional chain extenders are mixed then cured in a mold to form the finished article.
More commonly, the two step prepolymer process is used. In the first step a prepolymer is made by reacting the isocyanate with the polydiene diol and, optionally, a chain extending diol to form an isocyanate capped prepolymer. In the second step this prepolymer is then reacted with one or more amine curing agents and, optionally, a chain extending diol to form an isocyanate capped prepolymer. In the second step this prepolymer is then reacted with one or more amine curing agents and, optionally, additional isocyanate. This mixture is then cured in a mold to form the finished cast elastomer article. Cast elastomer articles are typically post cured to achieve final properties. Additional isocyanate is added in the second step to adjust the hard segment content of the elastomer, allowing a single prepolymer to be used to make materials with a wide range of hardnesses. The term hard segment content refers to the fraction of the composition that consists of the amine curing agent, optional chain extenders, and all of the isocyanate, both from the prepolymer and any added in the final step. The soft segment refers only to the polydiene polymer component. Because the final cast elastomer is crosslinked it is not necessary that the functionality of the polymeric diol or the isocyanate be exactly two.
The most common amine crosslinkers (curing agents) used in making cast elastomers are highly polar aromatic amines such as methylene bis(2-chloroaniline) (MCBA). When standard curing agents such as MCBA are used to try to make cast elastomers with the hydrogenated polydienes of the present invention, the materials produced have poor properties. When exposed to hot water aging, the properties improve rather than degrading, indicating poor initial cure.
It is an object of the present invention to provide cast polyurethane compositions with excellent properties using the hydrogenated polydiene diols of this invention. We have now found amine curing agents with low polarity form excellent cast elastomers with the diol/isocyanate prepolymers. We have shown that these materials have hydrolysis resistance far beyond industry standard materials.